Module 3.4: GPR Resolution

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GPR Resolution

Resolution refers to the smallest size object a sensor is capable of detecting.

In the context of cameras, or computer screens, it means dividing an area into more pixels so that each pixel is smaller. This allows for better definition of edges and the ability to pick out smaller features.

In the context of Ground Penetrating Radar, resolution refers to smallest target that GPR can detect and the ability to detect and differentiate between two separate targets.

The resolution is dependent on the size of the wavelength of the signal being transmitted – the smaller the wavelength the better the resolution will be and the smaller the targets that can be resolved.

The size of the wavelength depends on the frequency of the signal that the GPR is transmitting with lower frequency antennas transmitting a physically larger signal, and higher frequency antennas transmitting a physically smaller signal.

Some rules of thumb for the size of targets that can be detected using a GPR include:

  • 1/10th of the size of the wavelength of the centre frequency
  • ¼ of the size of the wavelength
  • A GPR target with a diameter of approximately 10% of the target depth
  • A target with a diameter of 1 inch for every foot of penetration.

Remembering that all of the above are approximations that are supposed to be easy to remember and give a guideline of GPR capability onsite, not exact figures.

Although they all appear different, some brief calculations shows that they all provide similar numbers:

  • A 200MHz antenna has a wavelength of approximately 0.5m in soil, 10% of this gives a 5cm minimum diameter target in optimal conditions.
  • The same 200MHz antenna will be wideband if it’s a GPR, that means it transmits frequencies from 100MHz up to 400MHz. A 400MHz signal has a wavelength in soil of approximately 0.25m. ¼ of 0.25 gives a minimum target size of 6.25cm in optimal conditions.
  • Using the 10% of depth rule, that 5-6cm diameter target might not be detectable using GPR if it is buried deeper than 50-60cm.
  • Using the 1inch diameter for every foot of depth rule, a 5cm diameter target (2 inches) might not be detectable if it is buried deeper than 60cm (2 feet).

The following examples show a GPR applied to a specific scene underground. It is intended to give a visual guide to the effect of different GPR antenna frequencies on the same ground, but are not technically accurate.

The scene includes:

  • Shallow and deep layers
  • Two shallow targets close together (two pipes for example)
  • A large deep target (such as void, sewer, basement, culvert)
  • An irregular shaped object (could be anything, just intended to show the differences between resolutions as regards the definition of the shape).
  • A row of shallow targets (intended to represent rebar’s within concrete)
  • A larger target beneath the row of rebar’s (intended to represent a utility beneath a concrete slab).

High Frequency GPR Survey

The following animation shows in a pictorial form what happens when a Ground Penetrating Radar with a relatively high frequency antenna is pushed above that scene.

GPR Resolution

Resolution refers to the smallest size object a sensor is capable of detecting.

In the context of cameras, or computer screens, it means dividing an area into more pixels so that each pixel is smaller. This allows for better definition of edges and the ability to pick out smaller features.

In the context of Ground Penetrating Radar, resolution refers to smallest target that GPR can detect and the ability to detect and differentiate between two separate targets.

The resolution is dependent on the size of the wavelength of the signal being transmitted – the smaller the wavelength the better the resolution will be and the smaller the targets that can be resolved.

The size of the wavelength depends on the frequency of the signal that the GPR is transmitting with lower frequency antennas transmitting a physically larger signal, and higher frequency antennas transmitting a physically smaller signal.

Some rules of thumb for the size of targets that can be detected using a GPR include:

  • 1/10th of the size of the wavelength of the centre frequency
  • ¼ of the size of the wavelength
  • A GPR target with a diameter of approximately 10% of the target depth
  • A target with a diameter of 1 inch for every foot of penetration.

Remembering that all of the above are approximations that are supposed to be easy to remember and give a guideline of GPR capability onsite, not exact figures.

Although they all appear different, some brief calculations shows that they all provide similar numbers:

  • A 200MHz antenna has a wavelength of approximately 0.5m in soil, 10% of this gives a 5cm minimum diameter target in optimal conditions.
  • The same 200MHz antenna will be wideband if it’s a GPR, that means it transmits frequencies from 100MHz up to 400MHz. A 400MHz signal has a wavelength in soil of approximately 0.25m. ¼ of 0.25 gives a minimum target size of 6.25cm in optimal conditions.
  • Using the 10% of depth rule, that 5-6cm diameter target might not be detectable using GPR if it is buried deeper than 50-60cm.
  • Using the 1inch diameter for every foot of depth rule, a 5cm diameter target (2 inches) might not be detectable if it is buried deeper than 60cm (2 feet).

The following examples show a GPR applied to a specific scene underground. It is intended to give a visual guide to the effect of different GPR antenna frequencies on the same ground, but are not technically accurate.

The scene includes:

  • Shallow and deep layers
  • Two shallow targets close together (two pipes for example)
  • A large deep target (such as void, sewer, basement, culvert)
  • An irregular shaped object (could be anything, just intended to show the differences between resolutions as regards the definition of the shape).
  • A row of shallow targets (intended to represent rebar’s within concrete)
  • A larger target beneath the row of rebar’s (intended to represent a utility beneath a concrete slab).

High Frequency GPR Survey

The following animation shows in a pictorial form what happens when a Ground Penetrating Radar with a relatively high frequency antenna is pushed above that scene.

The GPR energy is again represented as a vertical beam, we have already discussed that this is not accurate in real life – but it is easier to draw and represent in this diagram. The GPR is pushed across the surface and as it does so it repeatedly sends pulses into the ground (approximately 2.25cm between pulses).

Those pulses have a certain dimension, and this determines the smallest features which the GPR is able to distinguish.

It can be seen that the GPR energy from a high frequency antenna does not penetrate to see the deep target or the deep layer. However, it is able to detect the shallow layer, differentiate between the two pipes close together, and produce a good representation of the irregular shaped object. It is also (depending on the frequency) able to see the individual rebars and because some of the energy can fit between them, it can find the pipe beneath the rebar.

That scene might look something like the diagram below on the B-Scan.

Low Frequency GPR Survey

If we take a different, low frequency, Ground Penetrating Radar antenna and push it across the exact same scene we will recover slightly different results, even though the conditions below the ground are the same.

The low frequency antenna has superior penetration and is able to detect both the shallow and the deeper layer, as well as the large deep target. It also detects the two shallow pipes which are close together, but in this case it is unable to differentiate between the two of them so they will appear as one larger target.

The irregular shape is detected but it was not possible to produce as accurate a representation of the shape, and the rebar proved impervious to this GPR – so the target beneath the rebar could not be seen.

That scene might look something like the diagram below on the B-Scan.

Comparison between high and low frequency GPR results

If you put the two sets of GPR data side by side, you can see that they are similar but neither offers a complete and perfect picture of the below ground environment: High frequency antennas offer superior resolution at the expense of maximum penetration, and lower frequency antennas offer increased penetration, but less ability to resolve different features, and smaller features.

It is necessary for the user to choose the most appropriate GPR to achieve the objectives of the survey and to interpret the data in such a way as to produce the required results. In some cases it can be beneficial to use multiple frequencies for the survey.

GPR Wavelengths in Resolution

The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters, which is also the wavelength or a 1Hz signal.

The wavelength or a 1MHz radio signal is 299.8m (984 ft).

The following table shows the wavelengths for different GPR signals in air, and their approximate equivalents in soil (bearing in mind that a radio wave travels approximately three times slower in soil.

Approximate relative size of signal wavelength, for different GPR centre frequencies
Approximate relative size of signal wavelength, for different GPR centre frequencies

If we use the rule of thumb that under normal circumstances, a GPR is able to resolve targets approximately ¼ the size of its wavelength (and that it is a wide band antenna), we can produce the following table showing the approximate minimum size of target a particular frequency GPR can detect and an example of the types of features such a GPR might be suitable to detect.

Approximate relative maximum penetration for different GPR centre frequencies
Approximate relative maximum penetration for different GPR centre frequencies
Our Most Common Questions

Frequently Asked Questions

If your question isn't answered here, please do contact us for more information. 
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What can a Ground Penetrating Radar survey detect?

With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.

Will GPR compromise safety on my site?

GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.

Is a GPR survey 100% accurate?

While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.

Is GPR equipment difficult to use?

The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.

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